Suction sintering method and apparatus therefor

ABSTRACT

A suction-sintering charge of high permeability and stable structure is produced on a mobile suction-sintering grate or in a stationary or mobile suction-sintering pan by form compacting a moist material mixture (18) to be sintered to form a coherent cake (19), held together mainly by capillary forces, which cake is then broken into pieces having a size suitable to form a suction-sinter charge. The water content of the material mixture (18) is adjusted so that substantially the minimum of fuel is consumed at the intended sintering temperature and the compaction pressure is so adapted relative to the selected water content, corresponding substantially to the minimum fuel consumption, that the pore volume of the resultant cake is not totally filled by the amount of water present in said mixture.

This application is a continuation of application Ser. No. 406,235,filed Aug. 4, 1982 which was abandoned on Aug. 10, 1984.

The present invention relates to a method for producing a charge of highpermeability and stable structure on a mobile suction-sintering grate orin a stationary or mobile suction-sintering pan, comprising the steps ofcompacting a moist material mixture to be sintered, preferably betweenat least one pair of pressure-loaded rolls, to form a coherent cake, theparticles forming said cake being held together mainly by capillaryforces, and breaking the cake into pieces of a size suitable for forminga suction-sinter charge. The invention also relates to apparatus forcarrying out the method.

Suction sintering is primarily applied to the purpose of agglomeratingfinely divided iron-oxide material into a form suitable for charging toa blast furnace, and represents a substantial cost in the manufacture ofcrude iron. Consequently, it is important that as much sinter aspossible is produced per each unit of grate area, in order to reduceinvestment, maintenance, fuel and electrical-energy costs.

With regard to sintering capacity, the most important factor is thepermeability of the charge. The more gas drawn through the charge perunit of time, the less time required to effect a satisfactory sinteringoperation. The permeability of the charge can be increased in a numberof ways. The most usual method in this respect is to increase the amountof coarse particulate return-sinter admixed with the charge. Largequantities of coarse return-sinter, however, increase the amount of fuelrequired per unit of weight of final sinter, and incur additionalhandling costs. Furthermore, in certain instances it is necessary tocrush the final sinter, in order to obtain the requisite amount ofreturn-sinter. Another method often used is one in which the chargemixture is subjected to a subsequent rolling process in a drum. Althoughthe permeability of the charge is increased to a certain extent by thismethod, it is necessary to control accurately the amount of moisturepresent, and normally the amount of water needed is more than isjustified by the thermal progress of the sintering operation at thedesired low level of fuel consumption. Neither does rolling a mixedcharge in a drum produce a stable charge. Instead, the charge producedtends to become compacted when subjected to the high underpressuredesirable in suction sintering operations in respect of high plantcapacity, when sintering fine, particulate material.

In a number of cases attempts have been made to increase thepermeability of the charge by micro-pelletizing one or more of theiron-oxide products making up the charge. However, this techniquerequires the provision of additional and expensive pelletizingapparatus, such as drum or pan pelletizers. Only extremely fine-grainmaterial can be micro-pelletized, and normally it is necessary to use abinder, together with an accurately controlled addition of water, whichmay result in bogging in the lower regions of the charge during thesintering process, i.e. the mutual adhesion of respective particles inthe bottom region. Another drawback is that the charge constituents arenot mixed together with a thoroughness sufficient to favour desiredformation of slag during the sintering and blast-furnace operations.

In many parts of the world, iron-oxide is highly enriched in orderfirstly to increase the iron content and secondly to removecontaminents, such as phosphorus. In such working-up processes, prior tobeing enriched the ore must be ground down to such a small particle sizeas to render the resultant fine concentrate unsuitable for suctionsintering. Examples of such concentrates include the Mid-Swedish andNorthern-Swedish concentrates which have been purified of phosphorousand which are thus very fine. In order to find a market for suchmaterials, it has been elected to sinter them to pellet form. Sinteredpellets, however, are not ideal agglomerates for treatment in blastfurnaces, neither with respect to shape nor chemical composition, and inmany instances the sinter obtained from suction-sintering processes ispreferred, not least because it can be made self-fluxing, i.e. thesinter can be made to include constituents necessary for the blastfurnace.

In addition to fine particulate ore concentrates, there are many fineparticulate iron-containing materials which cannot, at present, beagglomerated by suction sintering techniques. Examples of suchconcentrates include pyrite cinders, dust from oxygen-blowing processes,and other very fine metallurgical intermediate products.

One attempt to find a solution with regard to a fine-particulate sintercharge is illustrated in Swedish Patent Specification No. 212 742,according to which a moist chargemixture is subjected to high-frequencyvibrations, to form a coherent cake which is placed in the form of smallpieces on a sinter grate and sintered. The aforementioned pieces arepassed through a screen and classified in a manner so that the coarserfraction of the charge lies nearest the grate and the finer fractionlies on top of said coarser fraction. Difficulties were encountered,however, when trying to put this method into practice. A relatively highmoisture content was required in order to obtain a plasticity suitableto form a cake. When the particulate material was vibrated, the surfacesof the cake became moist, which resulted in adhesion to the surfacesupporting said cake. In addition, it was difficult to achievehigh-frequency vibration of sufficient penetration depth.

An object of the present invention is to provide a novel andadvantageous method for producing a highly permeable suction-sintercharge of stable structure, by which the disadvantages discussed abovecan be at least substantially overcome.

To this end there is provided a method of the kind stated in theintroduction, which is further characterized by adjusting the watercontent of the material mixture to a level at which substantially theminimum of fuel is consumed at the sintering temperature intended; andby so adjusting the compaction pressure in relation to the set watercontent, corresponding substantially to the minimum fuel consumption, sothat the pore volume of the resultant cake is not totally filled by thewater present in said mixture. In this way, the aforementioneddisadvantages are overcome in a simple manner. In suction-sintering amaterial bed formed in accordance with the invention, there is formed,when the bed is ignited at the upper surface thereof a combustion layerwhich is thin relative to the thickness of the bed and which movesdownwardly through the entire bed while air is drawn by suction throughsaid bed. Air which has been heated by cooling the already heatedoverlying part of the bed is charged to said layer, while gas from thecombustion layer dries and heats the nearest underlying bed layer. Inthis respect, the water content is of great significance to the coursetaken by the sintering operation, since with the amount of air chargedper unit of time constant, the water content determines the speed atwhich the front of the combustion layer moves down through the bed. Onthe other hand, the speed of the cooling layer following the combustionlayer, and in which cooling layer the sintered charge material is cooledby the incoming air, is determined by the highest temperature reached inthe bed and the amount of air drawn therethrough, the speed of saidcombustion layer being constant when a constant amount of air issupplied and with a constant highest temperature. The combustion layerand the cooling layer, however, will only move at mutually the samespeed when the water content is that utilized in the method according tothe invention, whereat, as before mentioned, there is obtained a desiredthin combustion layer, which affords substantially minimum fuelconsumption at desired sintering temperature. If the water content isreduced, the speed at which the combustion layer moves will increase butnot the speed of the cooling layer, whereat the fronts of the two layerswill move apart and there will be obtained a thick combustion layer,requiring more fuel to be added in order to reach the desired combustiontemperature. The reason why, in this case, a reduced water contentresults in an increased fuel consumption is that the sinter containsmore residual heat when the combustion layer reaches the grate. If thewater content is, instead, increased above the value at which thecombustion and cooling layers move at the same speed, the speed at whichthe front of the combustion layer moves will decrease. The speed atwhich the cooling layer moves, however, is unchanged. Because of this,the sintering temperature will gradually fall, and may even fall to alevel at which the charge will be extinguished, and hence the higherwater content must be compensated for by increasing the amount of fuelsupplied. Thus, at every given sintering temperature there is adeterminable water content, which is adjusted in the method according tothe invention so as to obtain the lowest possible fuel consumption. Thecake formed by compacting the material in accordance with the inventionmay not, however, contain free moisture on the surfaces thereof, sinceotherwise the cake pieces become sticky and bake together in the chargeplaced on the grate, and when producing the cake by rolling, cause therolls to slip against the cake, and a mechanically weak cake isobtained. If the pores of the formed cake are unable to accommodate theamount of water necessary to provide for the lowest possible fuelconsumption, so that surface water occurs when pressing the material tocake form, it would seem an obvious thing to decrease the amount ofwater. This would mean that more fuel must be charged, however. Thus, ithas been found in accordance with the invention necessary to adjust thepore volume of the cake by regulating the compaction pressure such thatthe cake is able to accommodate the requisite amount of water. Theextent to which the pores are filled, however, must not be extensivelylow, since, to a large extent, it is capillary forces which hold thecake together. In this respect, it has been found that the pores of thepressed cake should be filled to at least 60%, preferably 60-90%.

In the conventional preparation of a sinter charge it has been foundnecessary, in many cases, to add more water than required from thethermal aspect, in order to obtain a permeable charge, which means thatmore fuel is consumed. This is not necessary when preparing a sintercharge in accordance with the method of the invention proposed here,which constitutes a highly important advantage.

If the pressed cake is too thick, there is a risk of the fuel, normallyfine coke particles, being entrapped so as to delay the ignition of thefuel. Because of this, the cake thickness should not exceed 12 mm, andshould not be smaller than 4 mm, since otherwise the cake will be tooweak to enable it to be broken up in a predeterminable manner intopieces having a size suitable to form a suction-sinter charge.Furthermore, in order to obtain good gas distribution and heat transferbetween gas and goods in the sinter charge, the compacted cake should bebroken up into pieces having a largest dimension of at most 20 mm,preferably of at most 15 mm.

The resistance to suction of a mixture of two particle sizes is greaterthan the total suction resistance of the two particle classificationsper se. Because of this it has been found to be of advantage when thecompacted cake, subsequent to being broken up, is classified in ascreening apparatus to form a relatively coarse particulate materialfraction, which is placed nearest the grate, and a fine particulatematerial fraction, which is placed upon the coarse fraction. In thisrespect, it is to particular advantage when the classifying process isadapted so that the major part of the material, preferably about 2/3rdsthereof, is present in the coarse material fraction lying nearest thegrate. The fact that the upper layer of the charge comprises the finerparticles, makes it easier to ignite the charge. This also improves thetransfer of heat between goods and gas at that stage of the sinteringprocess where stabilization of the front of the combustion layer isdesirable. In addition, fuel can be saved by mixing fine, particulateadditional fuel with the finer particulate fraction, since less fuel canthen be used for the remainder of the charge.

As before mentioned, the invention also relates to an apparatus forproducing on a mobile suction-sintering grate or in a stationary ormobile suction-sintering pan a highly permeable charge of stablestructure, said apparatus comprising means for compacting a stream ofmoist material-mixture to be sintered, to form a coherent cake, which isthen broken up into pieces of a size suitable to form a suction-sintercharge, said apparatus further including in accordance with theinvention means for so adjusting the compaction pressure in relation tothe water content of the material mixture that the pore volume of thecake formed is not completely filled by the water present in thematerial mixture. This arrangement enables the object of the inventionto be realized with the use of means of simple construction.

Further characterizing features of the apparatus and advantages affordedthereby are disclosed in the following claims and illustratedhereinafter.

The invention will now be described in more detail with reference to theaccompanying drawings, in which

FIG. 1 illustrates schematically a first embodiment of the invention,

FIG. 2 illustrates schematically a second embodiment of the invention,and

FIG. 3 illustrates schematically a third embodiment of the invention, inwhich bed layers are formed, one upon the other, by means of roll pairsarranged back-to-back in one and the same frame structure.

In FIG. 1 the reference 10 identifies a mobile suction-sintering rate,which is only partially shown, the bottom of the underlying suction box11 being shown at 12. Located on the grate 10 is a hearth layer 13 whichcomprises a coarse particulate material and which is intended to preventoverheating of the grate 10 during a suction-sintering operation. Itwill be seen from the drawings that as the grate moves slowly forwardsin the direction of arrow 15, there is gradually formed on the grate 10a bed 14 of material to be suction-sintered.

Arranged above the grate are means for compacting a moist materialmixture 18 arriving on a conveyor 17 to form a coherent cake 19, saidmeans in the illustrated embodiment having the form of a rolling millidentified generally at 16. The moist material mixture may, for example,comprise a fine particulate iron-ore concentrate, fine particulate,iron-containing metallurgical intermediate products, coke dust and slagformers, whereat the fuel and water content are adjusted with respect toone another so that substantially the minimum of fuel is consumed at therequisite sintering temperature. The conveyor 17 is provided with aweighing device 20 and is driven at a speed such as to supply a constantamount of material to the rolling mill 16 per unit of time. The stringof material passing to the rolling mill is smoothed out or levelled bymeans of a scraper device 21, so that said string or stream of materialhas a uniform thickness along that part of the rolls 22 and 23 of therolling mill active in compacting the material.

Connecting to the outfeed side of the roll pair 22, 23 is a slide platehaving a flat bottom 24, which forms a downwardly inclined support platefor receiving and guiding the cake 19 formed by the rolls 22, 23, to alocation directly above the grate 10. As the cake 19 passes over thelower edge 25 of the plate 24, the cake is broken, by the action ofgravity, into pieces having a size suitable to form a suction-sintercharge. The size of the pieces obtained can be varied by suitableadjustment between, inter alia, the amount of material mixture 18supplied per unit of time and the roll pressure, which parametersdetermine the cake thickness and the mechanical strength of the cake, atleast to a certain extent. The plate 24 may be arranged so as to enableit to be adjusted to different angles relative to the horizontal. Theangle at which the plate is inclined to the horizontal, however, shallbe small enough to ensure that the friction generated between the plateand the cake 19 will prevent the cake from being broken upon the plateby gravitational forces acting on said cake, before the cake reaches theedge 25, over which the cake is broken.

The rolls 22, 23 are driven synchronously by means of a respective motor(not shown) or a common motor, and are journalled in a rolling stand,which in the illustrated embodiment is supported by the ceiling of thebuilding housing the suction-sintering plant, a part of said ceilingbeing shown at 26. Only one rolling stand, identified by reference 27,is illustrated in FIG. 1. The other rolling stand has substantially thesame design. The lower roll 23 is journalled in a part 28 of eachrolling stand, while each end of the other roll 22 is journalled in onearm 29 of a double-arm lever pivotally mounted at 30 in the associatedrolling stand 27, the other arm of said lever being identified byreference 31. Acting against the other arm 31 is a hydraulicpiston-cylinder device 32, which endeavours to swing the lever in adirection in which the roll 22 is urged towards the roll 23. Adouble-arm lever with associated piston-cylinder device is provided foreach end of the roll 22. The levers may, to advantage, be rigidlyconnected together, so that they are swung synchronously. Further, thearrangement is such that the plane containing the axes of the rolls 22,23 is substantially perpendicular to the plate 24, so that the cake 19is supported by the plate 24 immediately it leaves the roll pair 22, 23,thereby eliminating the risk of unintentional, premature disintegrationof the cake.

A guide plate 33 which slopes downwardly from the plate 24 connects withthe lower edge 25 of the plate 24, over which edge 25 the cake 19 iscaused to disintegrate, said plate 33 being arranged to guide the cakepieces to a screening apparatus 34, where they are divided into arelatively coarse particulate fraction, which is placed nearest thegrate 10 on top of the hearth layer 13 to form a lower layer 35 of thecharge bed 14, and a relatively fine particulate fraction, which isplaced on top of the layer 35, to form an upper layer 36 of the bed 14.In this way, screening of the cake pieces is adapted so that the layer35 will contain approximately 2/3rds or more of the total amount ofmaterial in the bed.

In the illustrated embodiment, the screening apparatus 34 has the formof a drum comprising an outer casing of polygonal cross-sectional shapeprovided with screen openings. Extending along the whole of the lengthof the drum are outwardly projecting, substantially radial blades 37.The screening openings may comprise slots which pass through the drumcasing and extend peripherally thereof between mutually adjacent blades37. The width of the slots may, to advantage, be in the order of 10 mm.The drum is rotated clockwise by means of a motor (not shown), whereatcoarse cake-pieces falling onto the drum from the guide plate 33 willnot pass through the screening openings, but will be moved to the right,as seen in FIG. 1, by the blades 37 and fall down onto the grate 10, asindicated at 38, to form the bed layer 35. The smaller cakepieces willenter the drum through the screening openings and pass straight throughsaid drum, to form a stream 39 of relatively fine particulate material,said finer pieces being charged to the layer 35 already formed on thegrate, to form the upper charge layer 36. As illustrated in FIG. 1,desirable, fine additives, such as fine, particulate additional fuel,can be homogeneously mixed into the upper layer 36. To this end there isprovided a conveyor 40 on which the fine particulate additive material41 is conveyed to the screening apparatus 34, through which saidadditive material is caused to pass whilst being admixed with the finerfraction of the material arriving from the guide plate 33. As shown at42, the conveyor 40 is arranged to co-act with a weighing device, toenable correct metering of the additive material to the drum, and with ascraper device 43 for levelling out the stream of additive materialpassing to the screening apparatus 34, so that said stream of additivematerial is of uniform thickness over the whole of its width.

For the purpose of obtaining, by rolling, a cake 19 having the correctproperties there is provided a control means, generally referenced 44,by means of which the roll pressure can be automatically adjusted andmonitored so that the cake 19 obtains the correct porosity relative tothe amount of moisture present in the mixture 18, preferably so that thepores of the cake 19 are filled with water to a given percentage withinthe range of 60-90% by volume, within which range the cake obtains thedesired mechanical strength. The control means includes a pump 45, thesuction side of which is connected to a hydraulic liquid sump 46 and thepressure side of which is connected to each hydraulic piston cylinderdevice 32, via a line 47. A return line 48 extends from the line 47 backto the sump 46, there being arranged in the return line 48 a regulatablethrottle valve 49, the degree to which the valve 49 is throttleddetermining the pressure exerted by each piston-cylinder device 32 onthe arm 31, and therewith the roll pressure. The valve 49 is controlledfrom a known comparison circuit 51, via a control line 50, said circuitbeing arranged to receive on an input line 52 a signal which correspondsto a selected set-point value of the thickness of the cake 19. Thecomparison circuit 51 receives, via a line 53, an input signal sent by atransducer 54 arranged to sense the thickness of the cake 19, said inputsignal corresponding to the instantaneous cake thickness, whereat thecomparison circuit 51 compares the signals from the lines 52 and 53 andwhen the signal obtained on line 53 deviates from the valuecorresponding to the setpoint value of the thickness of said cake 19,adjusts the output signal on the line 50 so as to increase or decreasethe throttling effect exerted by the valve, thereby to change the rollpressure to a magnitude at which the desired cake thickness will beobtained.

As will be understood, the screening apparatus 34 and the conveyor 40can be omitted from the apparatus illustrated in FIG. 1 whenclassification of the pieces obtained when disintegrating the cake 19,and the supply of additive material 41 is unnecessary. Further, it is,of course, possible to control the manufacturing process of the sintercharge in a different way to that illustrated and described. It willalso be understood that the sintering grate 10 may be stationary and theapparatus producing the sinter charge arranged from movement along thegrate.

In order to improve the accuracy at which the material fraction 39 offiner particle size is placed on the grate when fractioning the materialby means of the screening apparatus 34, there is arranged inside thescreening drum a guide plate 55 which is journalled on the shaft 57 ofthe screening drum via end pieces 56, and the angle of inclination ofwhich guide plate can be adjusted by rotation about said shaft 57.

Coinciding or substantially coinciding elements in FIGS. 1, 2 and 3 areidentified with the same reference numerals and will not be described indetail in the following description of FIGS. 2 and 3, which illustrate afirst and a second modification of the apparatus shown in FIG. 1.

In the FIG. 2 embodiment there is arranged above the support plate 24adjacent the lower end thereof a driveable spiked roller 58, which ispreferably a variable-speed roller and which is intended to formfractural impressions in the cake 19 or to completely disintegrate thecake, the size of the cake pieces being determined by the distancebetween the spikes and the peripheral speed of the roller. For the sakeof clarity, the motor driving the spiked roller 58 and the meanssupporting said roller have not been shown in FIG. 2. The cake piecesobtained fall from the lower edge of the plate 24 in a stream 59, whichforms an upper layer 60 of the bed 14 on top of a bed layer 61previously placed on the grate 10 and the hearth layer 13. The bed layer61 may be formed with the aid of a further roll pair (not shown) locatedto the right, as seen in FIG. 2, of the illustrated roll pair, and thematerial of the layer 61 may have a different composition or a differentmean particle size to the material of the layer 60.

As will be understood from the aforegoing, when practicing the inventionaccording to the embodiment illustrated in and described with referenceto FIG. 2, two separate roll pairs 22, 23 and ancillary equipment,arranged at some distance apart, are required to build the bed layers60, 61 on the grate 10. While such an arrangement may be perfectlyadequate, it requires a relatively large amount of space and equipment.FIG. 3 illustrates an embodiment of the invention where the spacerequired has been cut down by combining the two pairs of rolls in acommon, compact unit.

Thus, FIG. 13 illustrates a mobile suction-sintering grate 10, on whichthe bed layers 60, 61 are formed by means of the two roll pairs 22, 23,and 22',23' arranged in one and the same open-frame structure 62,thereby obviating the need to provide two mutually spaced, separatelayer-forming stations and reducing the amount of space required. Asshown in the Figure, the open-frame structure comprises mutually spacedvertical members 63, top horizontal members 64 attached to ceiling 26and bottom horizontal members 65, only one such top and bottomhorizontal member being shown.

In the FIG. 3 embodiment, the roll pairs 22, 23 and 22', 23' arearranged back-to-back, so that the infeed sides of respective roll pairsface one another. As will be seen from FIG. 3, the infeed sides ofrespective roll pairs lie in a common storage vessel 66, into whichmaterial to be sintered is poured from a single conveyor 17. As with thepreviously described embodiments, the conveyor 17 is arranged to co-actwith a doctor on scraper means 21, for smoothing the material 18conveyed by the conveyor 17. Connecting with the outfeed side ofrespective roll pairs 22, 23 and 22', 23' is a support plate 24, whichis inclined at an angle at which, while allowing a cake string to bemoved down the plate 24 by the forces exerted thereon by the part of thecake issuing from the roll nip is not sufficiently large to cause thecake to slide down the plate under the influence of gravity at a speedliable to cause premature disintegration of the cake.

As shown in FIG. 3, each of the slide plates 24 has arranged in thevicinity of the end thereof remote from respective roll pairs a spikedroll 58 mounted for rotation on a bracket 67 carried by a respectivevertical member 63. Similar to the FIG. 2 embodiment, the spiked rolls58 are intended to break the coherent cakes on plates 24 into pieces ofgiven size, said size being dependent on the spacing of the spikes onsaid rolls and on the peripheral speed thereof.

The roll pairs 22, 23 and 22', 23' of the FIG. 3 embodiment are arrangedin a manner resembling the mounting of the roll pairs 22, 23 of the FIG.1 and FIG. 2 embodiments. Thus, the upper roll 22 or 22' of the rollstand shown to the left of the Figure is journalled for rotation in afixed stand part 28, while the lower roll 23 or 23' of said roll pair isjournalled on one arm 29 of a rotatable double-arm lever journalled at30 to the fixed stand part 28. The other arm 31 of respective double-armlevers is pivotally attached to the piston of a piston-cylinder device32, which endeavours to swing the lever in a direction in which therolls 23, 23' are urged towards the rolls 22, 22'. As with the roll pairof the embodiments previously described, the arrangement is such thatthe planes containing the axes of the rolls 22, 23 and 22', 23' aresubstantially perpendicular to respective slide plates 24.

Although not shown in FIG. 3 each of the roll pairs is governed by acontrol means 44 similar to that described with reference to FIG. 1.

In order to ensure that the particulate material 18 in the vessel 66 issufficiently friable to enable it to be picked up by the rolls of eachroll pair, there is provided means for agitating the heap of materialcontained by the vessel 66, either constantly or at given periods. Inthe illustrated embodiment, the agitating means comprises at least onepiston-cylinder device having on the distal end of the piston rod 68 ofsaid device a bar 69 which is parallel with the roll axes and isarranged to be moved into and out of the heap of material 18 containedby the vessel 66. The bar 69 is provided with surfaces of such shape andsize as to ensure that the material is stirred as the bar enters andleaves said heap, thereby to prevent bogging and to ensure a friablemass. As shown in the Figure, the cylinder 70 of the piston-cylinderdevice is secured to the frame structure 62 via a bracket 71. Inoperation, material 18 is fed to the vessel 66 by means of the conveyor71, and is kept in a friable state in said vessel by means of thereciprocatingly movable bar 69. The material 18 is then picked up by theroll pairs 22, 23, and 22', 23' and compacted therebetween to form acoherent cake having the porosity desired, said porosity beingcontrolled by said control means, as with the FIG. 1 embodiment. Eachcake is moved down its respective support plate 24 to a spiked roll 58,which disintegrates the cake into pieces of given size. The pieces thenfall onto the grate 10, forming either the layer 60 or the layer 61,depending on the position of the plate 24 from which said pieces fall,relative to the movement path of the grate 10, as clearly shown in theFigure. In order to provide a bed layer 61 of comparatively coarsematerial, the spikes of the spiked roll 58 associated with rolls 22',23' may have a wider spacing than the spikes of the other roll 58.

As will be understood, further modifications of the apparatus accordingto the invention are possible within the scope of the invention. Thus,the pair or pairs of rolls may be arranged obliquely to the longitudinaldirection of the grate or the rolling equipment may be separate from thesuction-sintering grate, whereat the finished sinter-charge material isdelivered to the grate by means of suitable conveying means, optionallyvia a charging or storage bunker.

The favourable effects which can be obtained by means of the inventionwill be evident from the following examples, in which all contents aregiven in percent by weight.

EXAMPLE 1

A material mixture comprising about 60% Grangesberg concentrate GAC(fine particulate concentrate having a high iron content and a specificsurface area of 550 cm² /g), about 32% of another concentrate having alower iron content, and about 8% slag former, of which 4% was burntlime, was admixed with 5% coke dust, 8% limestone and about 25% returnsinter having a particle size beneath 6 mm, to form a suction-sintercharge mixture. This mixture, which was composed to provide a highlybasic sinter, was moistened to a water content of 7%. The moist mixturewas rolled to form a cake whose pores were filled with water to about85% by volume and which was then disintegrated and screened in themanner described with reference to FIG. 1 and illustrated in saidFigure, there being used rolls having a diameter of 350 mm and a rollpressure of about 400 kp per cm of effective roll length. The screeningslots in the screening drum had a width of 10 mm, whereat about 30% ofthe material passed through the screening drum to form the finer upperlayer of the charge bed, the height of which reached to about 320 mmabove the hearth layer, which was 20 mm high. Sintering was effectedwith an underpressure of 1500 mm water column. The sintering time wasabout 10 minutes (including a standard addition of 2 minutes), therebeing obtained a production of about 42.5 tons per m² grate area and 24hours. When subjected to a tumbler test, the resultant sinter, which washard-burnt, had a value of about 60%. The aforegoing represents theresult of a number of sintering tests subsequent to obtaining the statedapproximate 25% balance between input and output return sinter. (For thetumbler test there was used a tumbler drum having a length of 900 mm anda diameter of 990 mm. The charge comprised 20 kg sintered materialhaving a particle size of 20-40 mm, and the drum was rotated 200revolutions during 8 minutes. Percentage by weight of tumbled materialhaving a particle size exceeding 6 mm was measured.)

A charge of the same composition but prepared in a conventional manner,without rolling, gave with all other sintering conditions equal aproduction of about 33 tons per m² grate area and 24 hours, i.e. aproduction which was almost 30% lower than the production obtained whenpracticing the method according to the invention.

EXAMPLE 2

A material mixture comprising about 60% Grangesberg concentrate GPC(pellet-sinter concentrate having a high iron content and a specificsurface area of 1600 cm² /g), about 32% of another concentrate having alower iron content and about 8% slag former, of which 4% was burnt lime,was admixed, for the purpose of forming a suction-sinter charge mixture,with 4% coke dust, 8% limestone and about 22% return sinter (<6 mm).This mixture was moistened to a water content of 7% and was rolled inaccordance with Example 1 above to form a cake, the pores of which werefilled with water to about 80% by volume, whereafter the cake wasdisintegrated by means of a spiked roller in accordance with FIG. 2 andlayed on a sinter grate, without being screened, to form a bed having aheight of about 350 mm over the hearth layer which was 20 mm high.

Subsequent to being dried, a sample of the material disintegrated bymeans of the spiked roller had the following particle size distribution:

    ______________________________________                                        Particle size (mm)                                                                            Percentage (%)                                                ______________________________________                                        >8              12.2                                                          8-6             26.2                                                          6-4             16.5                                                          4-2             16.3                                                          2-1             11.9                                                            1-0,2         10.8                                                          <0.2             6.1                                                          ______________________________________                                    

The sintering operation was carried out in complete accordance withExample 1 above, and required a time of 13 minutes (inclusive of theaforementioned two standard additional minutes) to carry out, therebeing obtained, subsequent to reaching the return balance of about 22%,a production of about 42 tons per m² grate area and 24 hours. A tumblertest as specified in Example 1 gave a result of about 65%.

It will be understood that such a fine concentrate as Grangesberg GPCcould only be incorporated in extremely small quantities, when preparingsuction-sinter charges in a conventional manner.

EXAMPLE 3

A material mixture comprising about 60% Malmberget concentrate MPC 3(pellet-sinter concentrate having a high iron content and a finenessapproximately the same as Grangesberg concentrate GPC), about 32% ofanother concentrate having a lower iron content, and about 8% slagformer, of which 4% was burnt lime, was admixed, for the purpose offorming a suction-sinter charge mixture, with 4% coke dust, 8% limestoneand about 23% return sinter (<6 mm). The mixture was moistened to awater content of 7% and rolled in accordance with Example 1 to form acake, the pores of which were filled with water to about 80% by volume,whereafter the cake was disintegrated by means of a spiked rolleraccording to FIG. 2, and then placed on a sinter grate, without beingscreened, to form a bed having a height of about 350 mm above the hearthlayer, which was 20 mm high.

Sintering was effected in complete accordance with Example 1 above, andrequired a time of 13.5 minutes (inclusive of the 2 minute standardadditional time), there being obtained, after reaching the returnbalance of about 23%, a production of about 39.5 tons per m² grate areaand 24 hours. The tumbler test as specified in Example 1 gave a resultof about 63%.

It will be seen that such a fine concentrate as Malmberget concentrateMPC 3 could only be incorporated in an extremely small quantity, whenpreparing suction-sinter charges in a conventional manner.

It will readily be seen from the above description and Examples that thepresent invention provides a novel method and a novel arrangement ofapparatus, capable of affording the following advantages:

1. Increased charge permeability, and therewith an increase inproduction per unit of grate area can be achieved, irrespective of theparticle size of the input material.

2. The moisture content of the charge mixture can be held at the lowlevel required for an optimal thermal sequence, thereby saving fuel.

3. When sintering, a small underpressure can be maintained whileachieving, at the same time, a high production, thereby reducing theamount of energy required to drive the fans.

4. The charge remains stable and will not compact during a sinteringoperation.

5. Sintering can be effected without admixing the charge with any otherreturn material than return sinter.

6. A low level of ignition fuel and fuel admixed with the sinter chargecan be maintained by employing a large charge-bed height.

7. Extremely fine particulate material, such as dust from steelmanufacturing processes, ash from coal combusting processes, etc., canbe sintered.

8. When compressing the charge mixture between rolls, only a low rollpressure is required, for example a linear pressure of 150-1000 kp,preferably 200-800 kp, per cm of roll length, thereby greatly reducingthe wear on the rolls.

We claim:
 1. A method for producing a moist suction-sinter charge ofhigh permeability and stable structure on a mobile suction-sinteringgrate or in a stationary or mobile suction-sintering pan, whichcomprises the steps of:(a) adjusting the water content of a moistparticulate material mixture to be suction sintered so that a minimum offuel is consumed to maintain the temperatures required for sinteringsaid particulate material mixture; (b) compacting said particulatematerial mixture to form a coherent moist porous cake having a thicknessof between about 4 mm to 12 mm, the compaction pressure being adjustedin such a manner that the water present in said mixture will fill thepore volume of said cake to 60-90%, said particulate material being heldtogether mainly by capillary forces; and (c) breaking said cake intopieces having a dimension of 20 mm or less to form said suction-sintercharge.
 2. A method according to claim 1, comprising classifying saidpieces obtained by breaking said cake in a screening apparatus to form arelatively coarse particulate material fraction, which is laid nearestthe grate, and a finer particulate material fraction, which is placed ontop of the coarse particulate fraction.
 3. A method according to claim2, comprising admixing fine particulate additional fuel with said finerparticulate material fraction.
 4. A method according to claim 1,comprising compacting said moist particulate material mixture between atleast one pair of pressure-loaded rolls.
 5. A method according to claim1, comprising breaking said cake into pieces having a largest size of atmost 15 mm.
 6. A method according to claim 2, comprising conducting theclassifying operation in a manner such that the major part of thematerial is included in said coarse particulate material fraction.